ARM: imx: imx8mn-*-evk: use reset-gpios in phy node

ARM: imx: imx8mn-*-evk: use reset-gpios in phy node

To be in sync with the linux devicetree change the 'phy-reset-gpios' in
the fec node to 'reset-gpios' in the phy node. The PHY reset will be
done by the eth-phy-uclass driver while probing the PHY. This is ok
since it is done before probing the fec.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Tested-by: Marek Vasut <marex@denx.de> # 8MNANOD4-EVK
Reviewed-by: Fabio Estevam <festevam@gmail.com>